1. Field of the Invention
The present invention relates to a method for the manufacture of UO.sub.2 nuclear fuels containing PuO.sub.2 in set amounts present as a solid solution with the UO.sub.2, and in the reprocessing of such nuclear fuel with nitric acid, the PuO.sub.2 is soluble in the nitric acid solvent.
2. Background of the Invention
In addition to uranium 235, the plutonium obtainable from burned-up fuel assemblies is known to be fissionable, also producing energy. Its best utilization is in so-called breeder reactors, in which over a certain period of time (doubling time) more plutonium is produced than the reactor itself consumes. But plutonium as dioxide mixed with uranium dioxide can also be burned up in so-called thermal reactors, producing energy. For this purpose, it is mixed with uranium as oxide in accordance with various methods and also used in various forms as nuclear fuel.
In the reprocessing of such nuclear fuels for obtaining the plutonium especially the breeder plutonium, difficulties are encountered, however, if the plutonium dioxide is not present as a solid solution with the uranium dioxide. This difficulty is due to the fact that it is not possible to dissolve as completely as possible the plutonium dioxide in the solvent HNO.sub.3 provided for UO.sub.2 fuel. Substantially complete dissolution of the plutonium dioxide is necessary for the reprocessing. This is possible only if additives, such as fluoric acid or hydrofluoric acid or fluorine ions or other additions containing dissolving ions, are admixed. In view of the corrosive action of the fluorine ion or of fluoric acid or also other dissolving ions on parts of the installation, as well as particularly also in view of the negative effects of adding fluorine ions in the glassification or encapsulation of the waste from the reprocessing, one endeavors to keep them but of the process.
A number of processes are known in principle, by which homogeneous solid solutions and soluble plutonium dioxide nuclear fuel can be produced, as for instance, melting the oxides UO.sub.2 and PuO.sub.2 ; the joint precipitation of different or similar compounds of uranium and plutonium; as well as also thorough milling of both oxide components, as described in the German Published Prosecuted Application No. 1 571 343.
However, the methods known to date are either too laborious and expensive such as, for instance, jointly melting the uranium dioxide and the plutonium dioxide at above 2000.degree. C., or the method is difficult to control and, in particular, requires the presence of an aqueous solution of the uranium and the plutonium, which is not a simple procedure. Furthermore, the residue insoluble in nitric acid which is left in most of the processes of joint precipitation, as well as in the dry milling processes known so far, is not small enough in all cases to ensure the reprocessing of burned-up fuel with high reliability without the addition of fluorine ions or the use of other measures. The reprocessing aspect, however, is of paramount importance for the future of nuclear reactor technology.
Experiments have shown that, for example, in the preparation of mixed oxides of 75% UC.sub.2+x powder and 25% PuO.sub.2 powder, which have been mixed by ball milling for several hours, and after sintering the powder, pressed into pellets, at 1700.degree.C. for four hours, an insoluble plutonium content of about 3%, referred to the plutonium input, was found. On the basis of these findings, it can be assumed as certain that the nuclear fuel prepared in accordance with German Published Prosecuted Application No. 1 571 343, column 3, lines 18 to 23, also does not have sufficient solubility, as there, the milling and sintering is likewise performed only once.
As found in the literature (e.g. Report of Oak Ridge National Laboratory No. ORNL/TM-5909), the results of dissolution tests on non-irradiated mixed oxides which were prepared by joint precipitation of ammonium diuranate and plutonium hydroxide and, further, processing into nuclear fuel pellets, scatter from 0.1 to 1% of the undissolved plutonium referred to the plutonium input.
In various investigations, it has also been found that a uranium-containing phase is present in the undissolved residue, in addition to a plutonium-containing phase. This result of the investigation shows that on the one hand, interdiffusion of the two compounds of the uranium and the plutonium has taken place, i.e. that pure plutonium dioxide is no longer present but that, on the other hand, there is a lower limit in the series of possible compositions of the uranium and plutonium dioxide, above which also completely homogeneous solid solutions of the uranium and plutonium dioxide upon subsequent reprocessing with nitric aicd results in insoluble plutonium oxide. On the basis of the presently known results, it can be assumed that this lower limit is at a composition of about 50% uranium dioxide and about 50% plutonium dioxide.
In this connection, it is worthy of note that it was found during the so-called post-irradiation investigation on mixed-oxide fuels irradiated in the reactor, that during the irradiation, demixing, i.e. separating out, of uranium and plutonium dioxide took place to a certain extent, with a preferred influence being observed in the temperature and stoichiometry gradient. This can have the result that, after irradiation, a higher concentration of plutonium is present in certain zones in the fuel rod than before the irradiation. Taking the investigation results mentioned above, into consideration, it can be expected in the case of mixed-oxide fuels with a plutonium content of more than 50% that fuels which were soluble prior to the irradiation exhibit insoluble components in certain zones after insertion into the reactor.